This invention relates to crystalline intermediates useful for preparing antibiotic compounds and to a process for obtaining the crystalline intermediates. In particular, it relates to 4-nitrobenzyl 7.beta.-phenoxyacetylamino-3-hydroxy-1-carba(dethia)ceph-3-em-4-carboxylat e in crystalline form and to the crystalline acetic acid solvate thereof.
The preparation of 1-carbacephalosporin antibiotics generally entails numerous steps, particularly if the synthesis is asymmetric. Examples of such preparative multi-step methods is the asymmetric method described by Evans, et al., Tetrahedron Letters, Vol. 26, pp. 3787-3790, 1985, and the method described by Hatanaka, et al., Tetrahedron Letters, Vol. 24, pp. 4837-4839, 1983. In such syntheses the intermediates in the individual steps are desirably obtained in highly purified form for use in succeeding steps. The crystallinity of intermediates reflects their purity and is highly desirable since unwanted side reactions involving impurities can be avoided in subsequent steps of the overall process.